The present invention relates generally to the field of marine electromagnetic surveying. Embodiments relate to a system and method for identifying degraded electrodes while the electrodes are deployed in a body of water, such as the sea, within a marine electromagnetic survey system.
Marine electromagnetic surveying may involve towing an energy source at a selected depth in a body of water. One or more survey cables also may be towed in the water at selected depths. The survey cables are essentially long cables which may have electromagnetic sensors disposed thereon at spaced apart locations. The energy source and survey cables may be positioned in the water by attached equipment, such as deflectors and position control devices. Actuation of the energy source may emit an energy field into the body of water. The energy field may interact with the subterranean formations below the water bottom. The change in the energy field due to the interaction with the subterranean formation may be detected by the electromagnetic sensors and used to infer certain properties of the subterranean formation, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons. In addition to towed survey cables, marine electromagnetic survey systems may also use sensors that are at a fixed location with respect to the energy source, which may include attachment of electromagnetic sensors on one or more cables positioned on the water bottom and/or attachment of the electromagnetic sensors to one or more subsurface acquisition nodes, for example.
Components used in a marine electromagnetic survey system may include electrodes. For example, electromagnetic sources and electromagnetic sensors may each include a pair of electrodes. When deployed for a marine electromagnetic survey, these electrodes may be exposed to seawater, which may undesirably interact with the surface of the electrodes. For example, because a potential difference may be typically applied across the electrodes during use, the surface of the electrodes may react with the seawater causing undesirable deposits and/or material loss that may degrade electrode performance. In addition, electrode performance may also be degraded by algae and other biological contaminations and/or growths that may occur on the surface of the electrodes. In some instances, the degradation of electrode performance caused by these undesirable interactions may limit the operational life of the electrodes used in the marine electromagnetic survey systems, as the interactions may result in decreased accuracy of measurements from the electrodes, for example, the electrodes may fail during operation. The current techniques for identifying electrodes which may be degrading may be to individually test electrodes above the surface of the water. Drawbacks to this technique for restoring performance may include the time, effort, and expense associated with retrieval and replacement of the electrode above the surface of the water.
Accordingly, there is a need for improved systems and methods for identifying electrodes that are degrading within marine electromagnetic survey systems.